Because written language can be read and re-read carefully, it must be more
precise than spoken language which isn't normally designed to be "played
back." This calls for a different set of values in written versus spoken
expression. One of those differences is that the phrasing, or choice of words,
used in written language must be very precise, much more so than that used in
oral communication. Consider the following:

A. Dialects. Spoken language is regional by nature. All across
the English-speaking world it's divided into dialects in which there are various
expressions and in which different words mean different things. For instance,
in British English "to knock up a girl" means "to knock on
the door of her house," whereas "to be stuffed" over there
is not "to eat too much" but means the same thing as "to be
knocked up" in American English. Thus, dialectal expressions like these,
if put in written form, can cause some confusion about what the author means
when they're read outside their native culture.

As a result, writers who aim at a readership beyond their immediate locale
must employ a more standardized language than they use in common parlance.
In writing, you should take special care to avoid regionalisms such as:

B. Prepositions. Watch how you use prepositions.* For instance,
things are different from (not "than") other things, or
connected to and with other things (not "between"
or "into"). In general, common sense should tell you which preposition
is right for which circumstance. Just visualize for a moment what you're saying,
and the right preposition will be readily apparent.

C. Technical Jargon. Don't write with "big words" or
technical jargon if you aren't sure about their correct usage. This usually
leads to comical misstatements, the likes of which teachers love to collect
and circulate. The classic example is: "Magellan circumcised
the world, with a one-hundred foot clipper." If you don't know the difference
between circumcising and circumnavigating, just say "sailed around."

All in all, it's impossible to list here all non-standard phrases that make
your writing unclear to general readers of English. As with spelling (see #12
below), try to identify the specific problems which phrasing presents for you
as you come across them.

*Prepositions are "little words," such as at,
of, by, toward, along, without,
around, under and so on.